Straightening the sides of this mesh
Go into Edit mode.  Click the vertex-select icon.  

Press C for circle select.
Color in a couple vertices you want to align, then right-click.  

Tap S to scale, then X, Y, or Z to limit along one axis.
You have to decide which axis is best.
Then press 0 to zero out that axis.  

If you notice movement along the wrong axis, no worries,
just click X, Y, or Z at this point to affect another axis instead.
Tap Enter to finalize command.  

Repeat with each pair of vertices 'till complete.